The present invention relates to intravascular devices deployable by minimally invasive techniques and, in particular, it concerns an intravascular filter which may be a stand-alone device or tethered to a guidewire, and is preferably easily retrievable.
The incidence of pulmonary embolism (PE) in the United States has been estimated at approximately 600,000 cases annually. Untreated PE carries a 30% incidence of mortality, which is decreased to 8% with anticoagulation. Although systemic anticoagulation remains the cornerstone of both treatment and prophylaxis for venous thromboembolism (VTE), permanent implantable endovascular filtering devices (i.e., caval filters, vena cava filters) are useful adjuncts for managing this disorder.
Although the concept of caval interruption to prevent embolization or propagation of proximal deep venous thrombosis (DVT) has been proposed since at least 1851, the first implantable endovascular devices for the treatment of VTE were the Mobbin-Uddin Umbrella and the Kimray-Greenfield filter. Like their modem counterparts, these devices were designed to filter and trap thrombi that could result in a lung emboluls. Their design allowed filtering to occur without occlusion of the venous return. A number of devices have since been introduced and original designs have undergone significant technical refinements. Most devices are made of fatigue-resistant stainless steel or titanium alloys and are compatible with magnetic resonance MRI techniques. In contrast to the surgical cutdown required to place early caval filters, nearly all filters now are deployed via a percutaneous catheter-guided method, under fluoroscopic guidance.
Vena cava filters (VCFs) are typically positioned within the infrarenal inferior vena cava (IVC) to trap thrombi arising from the lower extremities, avoiding potential occlusion of the renal veins. Limited reports also document the successful use of caval filters in the superior vena cava, as well as in the suprarenal IVC.
Both fatal and nonfatal complications have been reported for VCFs. Fatal or serious nonfatal complications are rare. Improved safety profiles and favorable experience with these devices have led a number of authors to advocate broader indications for the placement of caval filters, although many proposed indications remain controversial.
The number of VCFs placed annually has dramatically increased since the availability of the transcatheter delivery system, leading some authors to speculate that many filters may be placed without appropriate indications. Our experience in a major teaching hospital, consistent with many other reports, suggests that most VCF use is for what would be generally agreed upon as standard indications.
There are six permanent caval filters, representing four major design types, available for use in the United States. These are shown in FIG. 10 as: (A) stainless steel Greenfield; (B) modified hook titanium Greenfield; (C) alternating hook stainless steel Greenfield; (D) Bird's Nest; (E) Vena Tech; and (F) Simon-Nitinol.
The Greenfield filter (Medi-Tech/Boston Scientific Corp; Watertown, Mass.) was introduced in 1973. Three designs have been approved by the Food and Drug Administration for patient use in the United States. The original stainless steel cone-shaped design allowed 70 and 80% of the volume of the device to be filled with clot without a significant reduction in blood flow and was designed for a maximal caval diameter of ≦2.8 cm. The original stainless steel Greenfield filter was introduced through a relatively large 26F sheath and, due to its composition, led to significant artifact on MRI. It has been shown to be resistant to dislodgment at MRI field strengths of 1.5 T. This initial design was refined to a titanium “modified hook” Greenfield filter, which was contained within a smaller 14F sheath, facilitating percutaneous placement and causes no artifacts on MRI. The original stainless steel design was also recently modified to allow insertion over a guidewire through a smaller 12F sheath. It also has alternating hook arrangements. These two later designs may be safely accommodated within a larger caliber IVC.
The Gianturco-Roehm Filter, commonly known as the Bird's Nest filter (Cook Corp; Bloomington, Ind.), consists of two V-shaped struts supporting a random tangle of stainless steel wire. It was introduced in 1984. Stable placements of this filter in vessels up to 4 cm have been reported. The Bird's Nest filter is placed through a small sheath (14F), allowing for percutaneous placement through the femoral, internal jugular, or antecubital routes. One drawback of the Bird's Nest filter is a significant image artifact with abdominal MRI. Safety in a 1.5-T MRI field has been demonstrated with no significant device migration.
The Simon-Nitinol filter (Nitinol Medical Technologies; Woburn, Mass.) is introduced through the smallest sheath (10F) of all the designs available in the United States, allowing for introduction via an antecubital or the external jugular vein. This filter has a unique composition (nickel-titanium alloy) that assumes a preformed shape when warmed, but is pliable when cooled. This alloy is compatible with MRI and creates only minor local artifacts.
The Lehmann-Girofflier-Metais filter, referred to as the Vena Tech filter (B. Braun; Vena Tech; Evanston, Ill.) in the United States, is a derivation of a conical filtering device with anchoring longitudinal side rails. These serve to center the device in the vessel, thereby decreasing malalignment. The original design, introduced in 1986, was modified because of incomplete opening, caudal migration, and decreased clot trapping ability. The currently used Vena Tech cone and side rail lengths are approximately equal and are contained within a 12.9F sheath. The filter is made from an eight-metal alloy with a low ferromagnetic moment, which does not cause significant artifact on MRI.
The efficacy of caval filters may be affected by positioning. The filter may be malpositioned within the lumen of the IVC (i.e., tilted), thus reducing the effective filtering capacity of the device. All devices, with the exception of the Bird's Nest filter, arc subject to tilting. An in vitro study has suggested that clot trapping can be decreased in the Greenfield or Vena Tech filter if the degree of tilt is >15 degrees. The incidence of significant tilting of these two filter types has been reported as 1.7% and 1 to 2% respectively. On the other hand, Simon-Nitinol filters show no decrease in clot trapping efficiency when tilted up to 20 degrees. Although the reported cases are few, evidence is accumulating that recurrent PE after VCF placement may be associated with tilted devices.
The Bird's Nest filter, by virtue of its design, is subject to wire prolapse proximal to the anchoring struts. The incidence of wire prolapse is reported as 11%.
One of the factors thought to be responsible for the relatively slow adoption of intravascular filters into use is the non-retrievability of the available devices. This leads to problems during deployment where the initial alignment is incorrect, and generally requires the filters once deployed to be left in place indefinitely.
A further complication associated with these devices is penetration of tissues by the retaining hooks of the devices. Penetration of the retaining hooks of the filter through the lumen of the IVC is necessary for the proper anchoring of the device. Further penetration of these struts is commonly seen on radiographs, reported in at least about 10% of cases. In extreme cases, such over penetration may impinge upon adjacent organs, leading to serious or even fatal complications.
In an attempt to reduce risks of perforation, a number of recent developments halve employed rounded wire structures which become lodged within the blood vessel. Examples of this type may be found in European Patent Application Publication No. EP 121447, and U.S. Pat. Nos. 4,957,501 and 5,531,788. These devices all function primarily by asymmetric stretching of the blood vessel so as to reduce its cross-sectional area. The wire formations proposed are generally ineffective as filters.
A further problem of many devices to accommodate the relatively rapid variations in size of the blood vessels occurring during breathing, coughing, straining and the like. Failure to expand properly may lead to impaired filtering function and/or dislodging of the device. Failure to contract may damage the vessel wall.
In summary, all of the currently available intravascular filter devices suffer from one or more of a range of shortcomings including: risk of migration or dislodging of the device; risk of trauma to, or perforation of, the blood vessel; failure to accommodate variations in vessel size; and unreliable filtering performance.
Another field of application of the present invention is for distal prevention to avoid strokes. Stroke is a form of cardiovascular disease that interrupts blood flow to the brain. A stroke occurs when a branch of the carotid artery leading to the brain becomes clogged (ischemic stroke) or bursts (hemorrhagic stroke), preventing oxygen-rich blood from reaching the brain. As a result, brain cells die. Once dead, they do not regenerate which is why damage from a stroke is frequently permanent. Stroke accounts for 10% to 12% of all deaths in industrialized countries. For example, in a population of one million, 1,600 people will have a stroke each year, of which only 55% will survive six months post-stroke, and a third of the survivors will have significant disability. Stroke ranks third in terms of leading causes of death in the United States, behind heart disease and cancer. Strokes cause an estimated 150,000 deaths each year and are the leading cause of long-term disability. Current treatment options include medical management (drug therapy), carotid endarterectomy, or stent-supported carotid angioplasty. Carotid endarterectomy has demonstrated a marked increase in its use during the past two years on the basis of pivotal studies demonstrating a reduction in stroke after carotid revascularization. It is well documented that carotid endarterectomies have a 3% to 6% complication rate, depending if the patient is asymptomatic or symptomatic.
Embolization has represented an obstacle to widespread acceptance of stent-supported carotid angioplasty due to the brain's sensitivity to even small amounts of emboli, with clinically significant strokes occurring in the absence of angiographically definable branch vessel occlusions. If stent-supported carotid angioplasty is to compete effectively against the endarterectomy, it must demonstrate equal complication rates. In particular, carotid angioplasty must not lead to an increase in embolization or stroke rates.
Industry sources estimate roughly 100,000 carotid endarterectomies were performed in the United States alone in 1997. In the same year, approximately 90,000 procedures were performed internationally and are increasing at a faster rate than the United States. The desire among patients to have—and cardiologists to perform—less invasive procedures is evident. Industry estimates indicate that the number of carotid angioplasty procedures in the United States will grow from roughly 3,000 in 1998 to approximately 36,000 procedures in 2002. If the risk of embolization were reduced, this trend would develop much faster and fewer patients would require endarterectomy.
To reduce the risk of embolization during angioplasty, it has been proposed to deploy a temporary distal protection device associated with the end of a guidewire to catch any emboli resulting from the angioplasty procedure. The only commercially available devices offering such functionality are devices based upon inflatable balloons.
For completeness, reference is made briefly to U.S. Pat. No. 5,893,869 to Barnhart et al, which discloses a system for removing emboli from the blood flow. The device includes a conical filter funnel formed by an inwardly spiraling wire which funnels large particles towards an opening in the delivery catheter. The device is not a free-standing filter, being usable only while the catheter is inserted, and can only be used to trap emboli traveling towards the catheter.
There is therefore a need for an effective intravascular filter which could be deployed and removed by minimally invasive techniques, which would readily accommodate variations in size of the blood vessel, and which would minimize or eliminate the risks of perforation of the vessel wall.